1. Field of the Invention
The present invention relates to an encoder apparatus, more particularly to a rotary encoder apparatus having an incremental-type mechanism and providing an absolute-type rotation position signal.
2. Description of the Related Art
Encoder apparatuses are extensively used for position detection in movable machines in numerical control (NC) systems, industrial robot systems, etc.
There are two main types of encoder apparatuses: incremental encoder apparatuses and absolute encoder apparatuses.
An optical incremental encoder apparatus includes a rotary shaft for connection to a movable work machine whose position is to be sensed and a rotary disk fixed to the rotary shaft and having a light-coding region provided along a circumferential line of the rotary disk. The light-coding region is formed with a plurality of light-intercept portions and a plurality of light-transparent portions or slits. The light-intercept portions and light-transparent portions have the same width along the circumferential line and are alternatively arranged.
The incremental encoder apparatus further includes a light-emitting device, such as a lightemitting diode (LED), installed at a position opposing the light-coding region, and light-receiving devices, such as phototransistors or photodiodes (PD), located at another position opposing the light-coding region. The light-receiving devices face the light-emitting device across the light-coding region and slits of a fixed slit member installed between the light-receiving devices and the rotary disk.
The encoder apparatus also includes an electric circuit for outputting either an incremental pulse or a decremental pulse as a rotational position pulse in response to signals received from the light-receiving devices. A power source for activating the electric circuit is normally placed in a control unit away from the encoder apparatus.
The rotary shaft and the rotary disk rotate in response to movement of the work machine. The light emitted from the light-emitting device is received by the light-receiving devices through one of the light-transparent portions in the light-coding region and the slits in the fixed slit member. The electric circuit outputs an incremental pulse when the rotary shaft rotates in a forward direction and a decremental pulse when it rotates in a backward direction.
As clear from the above, incremental encoder apparatuses have simple constructions and a low number of structural elements. This facilitates assembly work and adjustment thereof. On the other hand, incremental encoder apparatuses suffer from the problem that, in the event of a power failure, the electric circuit cannot output incremental or decremental pulses. Despite the power failure, the work machine may still move, resulting in a discrepancy between the position value stored in the control unit and the actual position of the work machine. In such cases, when the power is restored, it is necessary to adjust the encoder apparatus using a reference position before normal operation can be resumed.
An optical absolute encoder apparatus includes a rotary shaft for connection to a movable work machine whose position is to be sensed and a rotary disk fixed to the rotary shaft and having a light-coding region. The light-coding region is formed with a plurality of concentric light-coding channels, each with a plurality of light-intercept portions and a plurality of light-transparent portions. Light-intercept portions and light-transparent portions are alternately arranged in each channel, the respective lengths of the intercept and transparent portions being defined by a predetermined relationship depending on the channel position. The light intercept-portions in the outermost channel on the rotary disk are usually equal in length to the light-transparent portions, so that a 1/2.sup.1 division pattern is defined. Similarly, the channel just inside the outermost channel defines a 1/2.sup.2 division pattern, and so on. The absolute angular position of the rotary disk is determined by the combination of transparent portions on a radial line in all channels. To obtain precise resolution of the angular position, concentric light-coding channels may be provided.
The absolute encoder apparatus further includes a plurality of light-emitting devices arranged in a line opposing a radial line of the rotary disk, each device facing a light intercept-coding channel, and light-receiving devices opposing the rotary disk on the other side; each device also facing a corresponding light-emitting device. The encoder apparatus also includes an electric circuit for outputting an angular position signal in an absolute form. Even in the absolute encoder apparatus, a power source is normally placed in the control panel away from the encoder apparatus.
Essentially, the signal output from the electric circuit represents the absolute angular position defined by the light received at the light-receiving devices through the transparent portions in the channels. This indicates the angular position of the rotary shaft and facilitates immediate restart after power failure without the troublesome reference position adjustment required in the incremental encoder apparatus. On the other hand, the absolute encoder apparatus has the disadvantages of a complex construction and troublesome assembly, meaning troublesome fine adjustment work.
In addition to optical encoder apparatus, there are also magnetic encoder apparatuses both incremental and absolute. These magnetic encoder apparatuses, however, have the same disadvantages as mentioned above.
In all the prior art encoder apparatuses, the light-emitting devices are always energized. Thus, there are the further disadvantages of high power consumption and, thus, a short service life of the light-emitting devices.